As will be appreciated, radio paging consists of a number of remotely located radio transmitters which are controlled from a master transmitter at a central originating location to provide a simultaneous broadcast or simulcast of information destined to be received by a portable pager usually carried on the person, with the pager providing either audio or digital information, or both. The utilization of multiple remote transmitters is required for reliable transmission in urban areas where the remote transmitters may be as close as 2 or 3 miles apart to accommodate a very dense urban environment in which one is concerned about building penetration. On the other hand the remote transmitters may be quite distantly spaced apart, perhaps 30 or 40 miles in rural areas.
One method of linking remote transmitters has involved telephone land lines between the originating central location and the remote sites, with simulcast phasing problems being solved through the utilization of programmed phase delays. These delays are calculated to accommodate lump delays associated with switching equipment, as well as delays associated with transmission over a lengthy cable network. However, while it is theoretically possible to compensate for variable phase delay, phasing throughout the network is difficult to achieve. Moreover, the expense of leased telephone lines is oftentimes prohibitive.
The primary method used for linking the central originating location with remote transmitting sites involves the utilization of RF link channels, in which the RF link channel is different from the paging channel utilized in the transmission of signals from the remote transmitting sites to the individual pagers.
It should be noted that link frequencies or channels are allocated between the so-called midband of 72 MHz and a newly legislated band centering around 900 MHz. Presently, the carrier frequencies for pager transmission is in bands varying between 35 and 931 MHz. In general, two different channels are used in normal terrestrial radio paging, namely the link channel and the paging channel.
The problem with an RF link system is overcrowding in which there are not enough link channels to satisfy demand for paging, and the fact that two channels of similar bandwidth are needed to provide throughput on one channel.
Note that most radio paging apparatus presently licensed are frequency modulated (FM) and that the width of the radio link channel as assigned by the FCC is typically between 20 to 25 KHz with appropriate guardbands. This means that the opportunities for a clear channel link are scarce and becoming even more scarce with the proliferation of paging services. Further, with a proliferation of radio paging services, not only have the paging channels been spoken for or allocated, it is also true that link frequencies are no longer available. Since land line and fiber optic linking systems are prohibitively expensive or technically difficult, link frequencies or channels are almost as valuable as paging frequencies or channels due to present overcrowding and the numbers of parties seeking FCC licenses.
In view of the increased usage of radio paging for not only voice messages but also digital data, the problem is one of frequency allocation. There is therefore a necessity for a system which will not increase the number of channels used while linking the remote transmitters, all the while not disturbing the entire body of technology related to simultaneous broadcasting.
The history of simultaneous broadcasting or simulcast has been one of reducing interference between two adjacent remote transmitters when both their signals are received by a pager in an overlapping broadcast area. Traditionally this problem has been solved by synchronizing the transmission from all remote stations to provide as close to an in-phase modulation condition as possible at all locations within the broadcast area. While FM capture is responsible for a pager locking out all strongest signal, problems exist especially in dense urban areas, with the problems being particularly severe with respect to audio transmission in which audio waveforms as received by the portable pager must be exactly in sync or very close.
In simultaneous broadcasting, it is not the carrier phase which is controlled but rather the phase of the modulation. In general, synchronization is accomplished through the interposition of a delay, typically in microseconds, with the individual delays at certain overlap points adjusted for optimal reception by pagers in the intended coverage area through adjustment of the simulcast delay parameters, given field data at various overlap points.
Since present simulcasting techniques permit successful receipt of audio, this also benefits the transmission of data to alpha-numeric pagers, since data transmission is significantly less sensitive to phase variation.
Typically, what has been accomplished in the past for RF linking is to provide a master link transmitter typically at the highest possible location such as on a mountain or tower or the like. RF links do not suffer from multipath distortion mainly because the antennas at both the master link transmitter and the remote transmitter are fixed with an unobstructed line-of-sight transmission. Moreover, fixed antennas can be of higher gain or can even be made directional to increase the signal-to-noise ratio. As will be appreciated, RF links do not suffer the same type of phase variability problems as telephone lines because antenna locations are fixed and the landline paths are not, due to variable paths through the telephone network.